In conventional mass spectrometry techniques, ions having mass-to-charge ratio within a selected range are isolated in an ion trap. The trapping field is then scanned to eject ions from the trap, in consecutive mass-to-charge order, for detection. For simplicity, throughout this specification, including in the claims, the phrase "consecutive mass order" will be used to denote consecutive mass-to-charge order.
Thus, trapped ions having mass-to-charge ratios m/z=37, m/z=38, m/z=39, and m/z=40, will be said to be ejected from a trap in consecutive mass order if they are ejected in either of the following two sequences: first m/z=37, then m/z=38, then m/z=39, and finally m/z=40; or first m/z=40, then m/z=39, then m/z=38, and finally m/z=37. However, the ions will be said to be ejected from the trap in non-consecutive mass order if they are sequentially ejected in the following order: first /z =37, then m/z=39, then m/z=38, and finally m/z =40.
In a class of conventional mass spectrometry techniques known as "MS/MS" methods, ions (known as "parent ions") having mass-to-charge ratio within a selected range are isolated in an ion trap. The trapped parent ions are then allowed, or induced, to dissociate (for example, by colliding with background gas molecules within the trap) to produce ions known as "daughter ions." The daughter ions are then ejected from the trap and detected.
For example, U.S. Pat. No. 4,736,101, issued Apr. 5, 1988, to Syka, et al., discloses an MS/MS method in which ions (having a mass-to-charge ratio within a predetermined range) are trapped within a three-dimensional quadrupole trapping field. The trapping field is then scanned to eject unwanted ions (ions other than parent ions having a desired mass-to-charge ratio) in consecutive mass order from the trap. The trapping field is then changed again to become capable of storing daughter ions of interest. The trapped parent ions are then induced to dissociate to produce daughter ions, and the daughter ions are ejected in consecutive mass order from the trap for detection.
U.S. No. 4,736,101 also teaches that a supplemental AC field can be applied to the trap during the period in which the parent ions undergo dissociation, in order to promote the dissociation process (see column 5, lines 43-62), or to eject a particular ion from the trap so that the ejected ion will not be detected during subsequent ejection and detection of sample ions (see column 4, line 60, through column 5, line 6)
U.S. No. 4,736,101 also suggests (at column 5, lines 7-12) that a supplemental AC field could be applied to the trap during an initial ionization period, to eject a particular ion (especially an ion that would otherwise be present in large quantities) that would otherwise interfere with the study of other (less common) ions of interest.
However, the conventional technique of ejecting trapped ions in consecutive mass order has several important disadvantages and limitations. During a prior art "consecutive mass order" scan, contaminating ions may unavoidably be produced in the trap, for example, as a result of undesired ion-molecule reactions, due to long storage times. Many of the resultant contaminating ions will not experience field conditions adequate to eject them from the trap, and will accumulate in the trap until their ion species is excited during the relevant portion of the consecutive mass scan. The contaminating ions' distorting effect on the data obtained during the scan becomes increasingly severe with elapsed time during a scan, to the extent that the concentration of contaminating ions increases with time. The inventive mass scan minimizes this distortion problem by enabling early excitation of selected trapped ions of interest, followed by subsequent excitation of other trapped ions of interest.
Another limitation of conventional "consecutive mass order" scanning is that the resolution obtainable inherently decreases with increasing ion mass-to-charge ratio, so that the quality of the data obtained during the early portion of a scan will typically differ significantly from that obtainable during the late portion of a scan.
Until the present invention, it was not known how to avoid both described limitations and disadvantages of conventional mass spectrometry methods in which trapped ions are ejected in consecutive mass order.